Conventionally, when a discharge lamp is lit by AC current, in order to prevent the discharge lamp from going out (i.e., becoming extinguished) unexpectedly, the lighting frequency is decreased in one cycle right after the start of the discharge lamp. This one cycle will be referred to as a DC period. As an electrode temperature of the discharge lamp is low right after the start of the discharge lamp, the electron emitting property for emitting electrons from the electrode is relatively poor right after the polarity is changed. Therefore, the electron emitting property is enhanced by increasing the duration of heating the electrode when the frequency is decreased below the frequency at the time of steady lighting. In this way, the change-over of the polarity at the first time can be accomplished easily. However, if the DC period is too long, the life of the discharge lamp is affected. Therefore, an upper limit of the product of the electric current in the DC period and the time is determined as the rating. This product of the electric current and the time will be referred to as the IT product. Concerning the cycle of the DC period, two systems are provided. In one system, the cycle of the DC period is always set constant regardless of the power source voltage, the state of the discharge lamp and the atmospheric temperature. In the other system, the cycle of the DC period is determined while the electric current and the time are being operated so that the IT product can become constant. For example, Japanese Patent Document JP-A-2002-216982 discloses a discharge lamp lighting circuit by which the DC period is determined when the IT product has reached a predetermined threshold value.
In the former system (the system in which the cycle of the DC period is constant), a scale of the control circuit can be reduced. However, in the case where the cycle of the DC period is short with respect to the electric power supplied to the discharge lamp, there is a possibility that the discharge lamp goes out unexpectedly right after the start of the discharge lamp. Usually, the DC period is determined by the rated electric power supplied to the discharge lamp. Therefore, in the case where the electric power supplied to the discharge lamp is reduced below the rated electric power because the power source voltage of the discharge lamp lighting circuit is decreased or the temperature of the discharge lamp lighting circuit is high, the electrode is not sufficiently heated in the DC period. Accordingly, there is a possibility that the discharge lamp goes out unexpectedly at the time of the first change-over of the polarity.
On the other hand, in the latter system (the system in which the cycle of the DC period is decided by the IT product), it is possible to solve the problems described above. However, it is necessary to execute the operation of the IT product. Therefore, a size of the control circuit for determining the DC period is increased. Accordingly, there is a possibility that the manufacturing cost is raised.